1. Field of the Invention
The present invention relates to a drive control apparatus for a hybrid vehicle whose drive shaft is rotated by an engine and an electric motor, and more particularly to a drive control apparatus for a hybrid vehicle which has a clutch disposed between an electric motor and a drive shaft, and engages and disengages the clutch depending on the state of the hybrid vehicle.
2. Description of the Related Art
In recent years, hybrid vehicles having drive wheels rotated by an internal combustion engine and an electric motor have been widely developed. The hybrid vehicle operates in various modes depending on the power source by which the hybrid vehicle runs. The various modes include a mode in which the hybrid vehicle is run by the engine alone, a mode in which the hybrid vehicle is run by the motor alone, a mode in which the output power of the engine is assisted by the motor while the hybrid vehicle is run by the engine, and a mode in which electric power is generated by an electric generator that is operated by the engine, and supplied to the motor either through a battery or directly to run the hybrid vehicle.
Various systems to drive hybrid vehicles have also been proposed. For example, one proposed four-wheel drive system drives either the front wheels or the rear wheels with an engine, or with both an engine and a motor, and drives the other wheels with the motor. Since the motor produces drive power (torque) for the hybrid vehicle, the drive power produced by the engine is reduced, and hence, lowering fuel consumption and exhaust gas emission. According to the above four-wheel drive system which drives either wheels with a motor, the drive power produced by the motor is transmitted to a drive shaft through a clutch. The motor has a speed reduction ratio that is set to a high value in order for the motor to provide high drive power. When the hybrid vehicle is running at high speed, the motor rotates at an extremely high rotational speed. Therefore, it has been proposed to disengage the clutch when the vehicle speed exceeds a predetermined level, for preventing the motor from overheating, and also for increasing the durability of the motor (see, for example, Japanese laid-open patent publication No. 2001-333507).
In the hybrid vehicle with the disengaging clutch, however, when the hybrid vehicle runs at a speed close to the predetermined level, then the clutch is frequently engaged and disengaged, thus, weakening the durability of the clutch and also making the occupants of the vehicle uncomfortable. For preventing the frequent engaging and disengaging of the clutch, it has been proposed to give hysteresis to the action of the engaging and disengaging clutch (see, for example, Japanese laid-open patent publication No. 2002-160541).
The hysteretic characteristic of the clutch is effective in reducing shocks that come from the engaging and disengaging of the clutch. Therefore, the structure of the clutch is simplified, the capacity is reduced, and the durability is increased.
When, however, the clutch is disengaged at a given vehicle speed, the motor will be generating a certain drive power, which is transmitted through the clutch to the drive shaft. Therefore, upon disengagement of the clutch, the drive power which runs the vehicle inevitably varies, also making vehicle occupants uncomfortable.
If the drive power upon the disengagement of the clutch is forcibly suppressed, it is difficult to achieve both a sufficient level of drive power and desirable hysteretic characteristics.
When the above four-wheel-drive hybrid vehicle picks up speed, the clutch is disengaged in a high speed range. The hybrid vehicle subsequently reduces its speed, or runs in a cruise mode, and the rotational speed of the motor is equalized to the rotational speed of the drive shaft, and the clutch is engaged. Since it takes some time to adjust the rotational speed, a certain time lag occurs until the motor is actually connected to the drive shaft by the clutch. Consequently, the time lag causes a delay for the motor to generate drive power, or to regenerate electric power. In order to eliminate such a delay, the clutch needs to be promptly engaged. If the clutch is engaged only by hysteresis, a range in which the motor is rotated without applying any drive power to the drive shaft widens, and the motor consumes more current and becomes less durable due to heat. Therefore, appropriated engaging of the clutch has not been proposed.
If two wheels are driven by the engine alone, it is preferable to process a control for stabilizing the vehicle when, for example, one of the drive wheels slips. When such a control process is adopted, the other wheels are driven by the motor, and hence the vehicle operates in a four-wheel-drive mode. The clutch is required to be promptly engaged in order to control the rotation of the wheels. The same problem as described above occurs when the vehicle slips.